Fluid purification

ABSTRACT

A purifier for water or air removes, reduces or detoxifies organic pollutants therefrom by causing the fluid to contact a matrix having surfaces with which a fixed anatase (TiO 2 ) or other photoreactive metal semiconductor material is bonded, in the presence of light of a wavelength that will activate the material.

This application is a division of application Ser. No. 094,000, filedSept. 4, 1987, now U.S. Pat. No. 4,892,712.

FIELD OF THE INVENTION

This invention relates to the purification of a fluid, for example wateror air, and more particularly to the removal, reduction ordetoxification from water or air of organic pollutants, such aspolychlorobiphenyls (PCB's), trihalomethanes, benzene derivatives,pesticides and others, some of which are mentioned below.

PRIOR ART

It has been known for some time that titanium dioxide can achievephotodechlorination of PCB's, as described by J. H. Carey et al in"Photodechlorination of PCB's in the Presence of Titanium Dioxide inAqueous Suspensions," "Bulletin of Environmental Contamination &Toxicology," Vol. 16, No. 6, pp. 697-701, 1976 Springer-Verlag New YorkInc. Carey et al describe irradiation by ultra violet light with awavelength of 365 nm of a 25 ppb aqueous solution of Aroclor 1254* inthe presence of suspended particulate titanium dioxide. After 30 min. nounreacted Aroclor could be detected in solution or adsorbed on thesurface of the TiO₂. Similar experiments were conducted with other PCB'sand resulted in an observed disappearance of the chlorinated biphenylsand the production of chloride ions. R. W. Matthews has reported theconversion (often called "mineralization") of a number of organiccompounds to carbon dioxide by exposure to near ultra violet light inaqueous suspensions of anatase, a form of crystalline titanium dioxide.The solutes studied were benzene, benzoic acid, benzoate ion, salicylateion, phenol, chlorobenzene, aniline, anilinium ion, nitrobenzene,chloroform and formic acid ("Carbon Dioxide Formation from OrganicSolutes in Aqueous Suspensions of Ultraviolet-Irradiated TiO₂. Effect ofSolute Concentration" by R. W. Matthews, Aust. J. Chem., 1987,40,001-000, pp 1-9). The same author had earlier reported similarresults with benzoic acid or sodium benzoate ("Hydroxylation ReactionsInduced by Near-Ultraviolet Photolysis of Aqueous Titanium DioxideSuspensions," J. Chem. Soc. Faraday Trans. 1, 1984, 80, pp 457-471).

Chen-Yung Hsiao et al have also reported the mineralization ofchloromethanes to CO₂ and HCl by the heterogeneous photocatalyst TiO₂("Heterogeneous Photocatalysis:Degradation of Dilute Solutions ofDichloromethane (CH₂ Cl₂), Chloroform (CHCl₃), and Carbon Tetrachloride(CCl₄) with Illuminated TiO₂ Photocatalyst," Journal of Catalysis 82,1983, pp 418-423).

Similar reactions have not been limited to TiO₂. Other metalsemiconductors, such as ZnO, CdS, WO₃ and SnO₂, have been utilized inphotocatalytic processes for the degradation of environmentalcontaminants ("Photocatalysis Over TiO₂ Supported On A Glass Substrate,"by N. Serpone et al, Solar Energy Materials 14(1986) pp 121-127,Elsevier Science Publishers B.V.-North-Holland Physics PublishingDivisions, Amsterdam).

SUMMARY OF THE INVENTION

The object of the present invention is to adapt this previously observedlaboratory reaction to a practical fluid purification system for eithercommercial or domestic use, e.g. for the purification of potable wateror for treating industrial wastes or environmental spills, or forremoving pollutants from air.

Most of the laboratory reactions discussed in the literature referred toabove involve forming a suspension of particulate TiO₂ (or other metalsemiconductors) in the water containing the organic pollutants that areto be converted into harmless by-products. This procedure isimpracticable for the commercial purification of water, because theparticulate TiO₂ would have to be subsequently removed before the watercould be used, and this would be either impossible or prohibitivelyexpensive.

In its preferred form, the present invention solves this problem byfirmly bonding the TiO₂ or other metal semiconductor (subsequentlyreferred to as the ("photoreactive material") with, to or into surfacesof a substrate that has the properties of a large surface area forcoating with the photoreactive material, a porous construction such thatthe fluid to be treated can thoroughly contact the coated surfaces, andsufficient transparency to light at a wavelength to which thephotoreactive material photoreacts to ensure that all the coatedsurfaces receive such light at an adequate energy level to ensure thecatalytic or photoreactive effect.

The bonding of the photoreactive material to the substrate must be sofirm that no appreciable amount of it enters the fluid.

The preferred form of base material for use as the substrate is a lengthof fiberglass mesh that has been wound into a cylindrical, multi-layersleeve having a number of convolutions superposed on each other.

Fiberglass absorbs only a small percentage of the light at thewavelengths used, i.e. between 300 and 425 nm, and hence such mesh issubstantially transparent. The sleeve is preferably such that the holesin the mesh of one convolution are oriented out of alignment in relationto the holes in the next convolution.

Combined with the facts that a large proportion of a typical mesh isholes and that the strands absorb only a small amount of the lightpassing through them, such a non-aligned orientation enables the lightto pass through multiple convolutions with relatively littleattenuation. As a result, a sleeve of such mesh placed around acylindrical lamp can have a large number of superposed convolutionswithout reducing to an unacceptable level the intensity of the lightreceived by the photoreactive material coated on the outer convolution.This ability to use a multiple layer sleeve of mesh as a substrate toform a matrix greatly facilitates the achievement of a compactarrangement.

The mesh need not necessarily be wound into a sleeve. The convolutionsreferred to above can be replaced by superposed layers of any otherphysical shape, such as a series of concentric surfaces. For example,coated films could be rolled into coaxial tubes, or coated glass (e.g.sintered or porous glass) tubes of different diameters could be insertedwithin each other to yield a series of concentric surfaces. A singlesuch tube can also be used. While the tube need not be porous, i.e.could be imperforate, there is the advantage of a porous tube or tubesin providing a larger surface area for the coating.

Moreover, the mesh need not necessarily be made of fiberglass. Any otherassembly that is sufficiently transparent to the light can be used. Suchan assembly can achieve its transparency either from its structure, e.g.a very open mesh, or from its own intrinsic property. For example, amaterial like stainless steel that is itself opaque to the light can beused, provided it has a sufficiently open structural form, i.e. thesubstrate as a whole is sufficiently transparent. What constitutessufficient transparency of the substrate will depend on how many layers(convolutions) are superposed on one another. While the use of othermaterials is not precluded, fiberglass will normally be the preferredchoice among currently available materials from the viewpoints ofcheapness, light weight, convenience of handling, relatively hightransparency and its inertness to the reactants.

Moreover, it is not essential to use a mesh. The invention is operablewhen the photoreactive material is in the form of a bonded coating onany suitable substrate.

Also, the substrate may take the form of filamentous fiberglass, i.e.non-woven insulation type material, or woven or spun glass wool, inwhich case the base material would not form distinct layers orconvolutions but would simply constitute a loosely packed mass offibers.

To ensure a clear understanding of the terminology adopted herein, it isto be noted that the term "base material" refers to the actual materialitself, e.g. fiber-glass; "substrate" refers to the assembly of thismaterial, e.g. as a loose mass of fibers or as a mesh; while "matrix"refers to the combination of the substrate with the photoreactivematerial bonded therewith.

Assumihg that anatase is the chosen photoreactive material, thepreferred method of bonding it to the surfaces of the matrix is by anadaption of the known sol-gel technique. See, for example, "Use ofSol-Gel Thin Films in Solar Energy Applications" by R. B. Pettit et al,Solar Energy Materials 14(1986) pp 269-287, Elsevier Science PublishersB.V.-North-Holland Physics Publishing Division, Amsterdam.

As applied to the preferred form of the present invention, this bondingtechnique can involve the following steps.

A titanium alkoxide, e.g. ethoxide, dissolved in an organic solvent,e.g. anhydrous ethanol, is reacted with a controlled amount of an acid,e.g. nitric acid and water, to form a coating solution. Other methodscan be adopted for forming a coating solution, the important feature ofwhich is that it should contain a titanium alkoxide. The fiberglass meshis then dipped into such a coating solution, the process being carriedout in dry conditions. Subsequent exposure of the coated mesh to airresults in a controlled hydrolysis process, i.e. causes setting of thepolymer to yield an amorphous TiO₂ layer on the mesh surfaces. After adrying period, the coated mesh is fired at about 400° C. for about onehour, which converts the amorphous layer to the anatase crystalstructure necessary for the photocatalytic reaction to proceed. Theresult is a very tight bonding of the anatase layer with, to or into thefiberglass strands, hence avoiding any measurable amount of the anataseentering the fluid to which the matrix will subsequently be exposed.This bonding may involve a covalent bonding between the substrate andthe anatase.

Additional details of a bonding technique are given below.

As mentioned above, the invention is not limited to use with TiO₂. Otherphotoreactive metal semiconductors can replace the TiO₂, such as CdS,CdSe, ZnO₂, WO₃ and SnO₂.

The sol-gel technique can only be used to apply metal oxides. Thenon-oxide semiconductors must be applied by some other means, such as byvacuum or vapor deposition, electroplating or sintering. In this regard,it is important to note that sol-gel is the only one of these methodsthat is usable with non-flat surfaces. Consequently, sol-gel allowsfabrication of the substrate in its final mechanical configurationbefore coating. With the non-oxide semiconductors, the method ofmanufacture must be changed. The base material will first have to becoated with the photoreactive semiconductor and then fabricated into amatrix.

Enhanced results can sometimes be achieved by doping the active materialwith a suitable dopant, e.g. platinum.

As indicated above, while the matrix can take a wide variety of physicalforms, it is of substantial practical value to have a matrix that is ofsuch a nature that it can readily be penetrated by the fluid, so thatthe latter comes into contact with substantially all the surfaces. Tothis end, it is desirable that the matrix encourage turbulent flow ofthe fluid through itself, a result that is well achieved by a rolled-upsleeve of fiberglass mesh.

Hence the invention, in one aspect, consists of a matrix for use in amethod of removing, reducing or detoxifying organic pollutants from afluid, comprising a porous substrate having a photoreactive metalsemiconductor material bonded with, to or into surfaces of saidsubstrate.

The substrate is preferably at least partially transparent to light at awavelength to which the semiconductor material photoreacts.

The invention also consists of a method of removing, reducing ordetoxifying organic pollutants from a fluid, comprising bringing suchfluid into contact with a photoreactive metal semiconductor materialbonded with, to or into surfaces of a substrate while subjecting suchphotoreactive material to light of wavelengths that activate saidmaterial.

The invention also consists of apparatus for carrying out this method.

Finally, in another aspect, the invention consists of a method ofmanufacturing a matrix for use in a method of treating organicpollutants from a fluid, said method comprising (a) preparing a coatingsolution containing a titanium alkoxide;

(b) coating surfaces of a base material with said coating solution toproduce an amorphous layer of titanium dioxide on said surfaces; and (c)firing the coated base material at an elevated temperature to convertthe amorphous layer to a layer of anatase bonded to the base material.

BRIEF DESCRIPTION OF THE DRAWING

A specific example of the invention is illustrated in the drawings, inwhich

FIG. 1 is a perspective view of a matrix; and

FIG. 2 shows an example of how such a matrix can be used in practice.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The matrix is a mesh substrate consisting of two transverse series offiberglass strands 10 with, to or into which anatase or otherphotoreactive semiconductor material (not visible in the drawings) isbonded. This mesh is wound into a sleeve 11 to form superposed layers(convolutions) 12, 13, 14 etc. In a typical example, there could bebetween ten and twenty such convolutions, although this number can varywidely as circumstances require. An individual strand 15 threaded in anaxial direction through a series of holes in the mesh serves to securethe end 16 of the outer convolution 12 to the next convolution 13 below.In addition, individual strands 17 are threaded circumferentially(preferably in a wavy fashion) around each convolution at each end ofthe sleeve and at its longitudinal centre (only the upper strand 17 isshown). These strands 17 act as spacers in serving to prevent theconvolutions from lying too tightly against each other. In other words,they help to maintain the looseness and hence the porous property of thesleeve whereby water or air can flow along the matrix between theconvolutions and thoroughly contact the coated surfaces. Obviously,other means can be used for securing the convolutions of the sleevetogether while ensuring adequate spacing between them. For example,these strands 15 and 17 can be replaced by an inert, high temperature,inorganic adhesive applied as beads between strands 10.

The preferred method of bonding anatase to the fiberglass strands of amesh is as follows:

1. Preparation of Titanium (IV) Alkoxide Solution in Alcohol

Two solutions are prepared. In solution A, distilled water and 70% HNO₃are mixed into anhydrous ethanol. In solution B, titanium (IV) ethoxideis dissolved in anhydrous ethanol. Solution B is added slowly tosolution A with adequate stirring to ensure complete mixing of thesolutions. The mole ratios (to titanium ethoxide, Solution B) are:

    ______________________________________                                        Solution A:      Solution B:                                                  ______________________________________                                        water     1.93       titanium ethoxide                                                                           1.00                                       nitric acid                                                                             0.02       ethanol (anhydrous)                                                                         40.6                                       ethanol   11.8                                                                ______________________________________                                    

The above operations must be performed in a dry atmosphere, sinceexposure of the titanium ethoxide to water vapor will result inpremature hydrolysis of the ethoxide. The final solution containing 30gm/1 of TiO₂ is aged overnight (about 16 hours) before being depositedon the base material.

2. Fiberglass Mesh Cleaning Procedure

1. Heat in oven at 400° C. for one (1) hour

2. Soak in 1% (weight/weight) NaOH solution at 85°-95° C. for 10 minutes

3. Soak in iN HCl at 40°-60° C. for 30-120 seconds

4. Rinse with isopropyl alcohol

5. Dry in oven at 100°-200° C. until ready to dip

3. Coating of Base Material

The fiberglass mesh is dipped into the titanium alkoxide solution forapproximately one (1) minute and withdrawn almost vertically at ratesranging from 20-80 cm/min. The coated material is dried at roomtemperature for 1-2 hours in a dust free environment before heattreatment.

4. Heat Treatment

The coated material is treated at 400° C. as follows:

Heating rate=room temperature to 400° C. in 2-5 hours

Soak time=1 hour at 400° C.

Cooling rate=400° C. to room temperature in 5 hours or more

While the foregoing process has been found effective, many variationsare possible. In particular, it has been found that the soakingtemperature will vary with the amount of acid, and therefore atemperature within the range of 350° to 600° C. may be used.

FIG. 2 shows diagrammatically how the sleeve 11 can be mounted around acylindrical lamp 18 inside a cylindrical jacket 19 (conveniently, butnot necessarily, of stainless steel) having an inlet 20 for pollutedfluid and an outlet 21 for the treated fluid from which the pollutantshave been substantially eliminated by conversion to harmlessby-products.

The device in which the matrix carrying the fixed photoreactive materialis caused to be contacted by the fluid can vary widely in size, shape,and general arrangement.

The lamp will be such as to emit light with wavelengths less than 425nm, preferably peaking at 340-350 nm. It is notable that this range ofwavelengths is present in the sunlight that reaches the surface of theearth, which opens up the possibility of dispensing with the lamp inlocations where a source of power is unavailable. This result could beachieved by making the jacket 19 of a material transparent to light atthese wavelengths, or otherwise arranging to expose the photoreactivematerial to sunlight in the presence of the water to be purified.

Such a transparent jacket could also be used in an arrangement in whichone or more lamps are located outside the jacket.

In addition to the conversion of organic pollutants to carbon dioxide,and other harmless by-products, the light may simultaneously achievesome antibacterial function. Conventional ultra violet sterilizers usewavelengths around 250 nm, because this is the so-called germicidalwavelength at which the two strands of the DNA helix become lockedtogether to prevent the bacteria reproducing. However, at the longerwavelengths used in the present invention, e.g. between 300 and 425 nm,and given a sufficient intensity of light, the bacteria can be expectedto experience direct destruction.

While the form of the invention illustrated is a continuous flow-throughprocess, the invention is equally applicable to batch processing.

Advantages of the device disclosed herein include the use of a matrixwith

(a) a high surface area;

(b) transparency to the light throughout all areas of the matrix andhence effective utilization of the light by the photoreactive material;

(c) a high porosity and hence a low fluid pressure drop;

(d) an effective turbulent mixing of the contaminated fluid as it passesthrough the matrix in contact with the photoreactive material; and

(e) good resistance to clogging, due to the use of an open mesh-likestructure, in contrast to a filter with much smaller holes.

We claim:
 1. A matrix for use in a method of removing, reducing ordetoxifying organic pollutants from a fluid, comprising a substrate inthe form of a plurality of layers of a filamentous, fibrous or strandedbase material, and a photoreactive metal semiconductor material bondedwith, to or into surfaces of said layers, at least one of said layersbeing at least partially transparent to light at a wavelength to whichthe semiconductor material photoreacts.
 2. A matrix as in claim 1,wherein said layer transparency is a result of at least one of(a) thebase material being substantially transparent to light at saidwavelength, and (b) the layer having a porous nature permitting thetransmission of said light therethrough.
 3. A matrix as in claim 2,wherein the base material is fiberglass and the layers are porous.
 4. Amatrix as in claim 1, wherein said layers are formed by convolutions ofa cylindrical, multi-convolution sleeve.
 5. A matrix as in claim 1,wherein said layers comprise at least two coaxial tubes of film or glasscoated with said photoreactive material.
 6. A matrix as in claim 5,wherein each said tube is imperforate.
 7. A matrix as in claim 1,wherein the photoreactive material is anatase.
 8. A matrix as in claim1, wherein the photoreactive material is selected from Cds, CdSe, ZnO₂,WO₃ and SnO₂.